Now that extract-strings has moved to webapp, remove it from khan-exercises.

Summary:
It will always live in our hearts (and in the git history), but it's
trouble to have two copies of the same code lying around, so let's get
rid of the one that's no longer used.

Test Plan: None

Reviewers: john, chris

Reviewed By: chris

CC: chris, emily, alpert

Differential Revision: http://phabricator.khanacademy.org/D2568